1. Field of the Invention
The present invention relates to an image processing apparatus which performs screen processing on an input image.
2. Description of Related Art
Conventionally, when a halftone image is output by an image forming apparatus such as a copier, printer, etc., screen processing using a threshold matrix is performed (see Japanese Patent No. 3726470). Japanese Patent No. 3726470 discloses an image processing apparatus in which after calculating an output pixel value based on a halftone dot pattern with a higher resolution than the resolution of the input and output image signal, a multivalued signal adjusted to an output resolution is obtained.
An example of conventional screen processing using a threshold matrix of a high resolution is described with reference to FIG. 8. In the example here, the resolution of the input image and the output image is 1200 dpi (dot per inch) in both the horizontal scanning direction and the vertical scanning direction, and the resolution of the threshold matrix is 2400 dpi in both the horizontal scanning direction and the vertical scanning direction.
First, gamma correction processing is performed on the input image of 1200 dpi and then the input image after gamma correction processing is converted to the same resolution (2400 dpi) as the threshold matrix. Since in a conversion from 1200 dpi to 2400 dpi the resolution is doubled in both the horizontal scanning direction and the vertical scanning direction, data for four pixels is generated from one pixel. Next, screen processing is performed on each pixel after conversion of resolution using a threshold matrix of 2400 dpi. Then, the result (2400 dpi) output by the screen processing is converted to the resolution according to the resolution (1200 dpi) of the output device.
However, in order to perform screen processing after converting the input image to the resolution which is the same as the threshold matrix, screen arithmetic circuits are necessary for the number of pixels corresponding to the resolution conversion. For example, when the resolution of the image is converted from 1200 dpi to 2400 dpi, 4 screen arithmetic circuits are necessary in order to process in parallel data of 4 pixels. Similarly, when the resolution is converted from 600 dpi to 2400 dpi, 16 screen arithmetic circuits are necessary in order to process in parallel data of 16 pixels. Therefore, when using a method of converting the input image to a resolution with the same resolution as the threshold matrix of a high resolution, the scale of the circuit of the image processing apparatus becomes large. Consequently, computational cost increases and practical use is difficult.